Over 250,000 hip replacements are inserted annually in the U.S.A., with aseptic loosening the greatest source of long-term failure. Since femoral component subsidence measured on plain radiographs (accuracy 2-3 millimeters) is not typically evident before five years, lengthy follow-up is required to determine whether a new design reduces failure rates, and thereby widens patient indications and reduces complicated revision operations. Radiostereometry (RSA) sensitively (100-500 micrometers) tracks implant position. It is arguably the gold standard for detecting the early excessive subsidence shown to be a reliable predictor of ultimate clinical failure. However, implementation of RSA requires custom components and complex radiographic equipment, and is not widely available. We propose to develop an implantable, non-contacting sensing plug, to be inserted just distal to the tip of a femoral stem. External RF powering and embedded digital telemetry remotely query the plug to resolve subsidence to less than 100 micrometers, allowing the hallmark early subsidence (and thereby poor designs) to be detected within the first two years. Recoverable implant motion can be monitored real time (update rates of 200 Hz). The implanted microelectronics will include non-volatile memory and bi-directional wireless communications, allowing the plug to measure and maintain a record of subsidence over time.